<p>No one has really given the op an adequate explanation credits. As sax said, they generally represent the number of hours that you’re in class for that course each week. The way each institution does it is different, but a general way to look at it is:
3 credits - many social science and humanities courses
4 credits - most math and science classes (to account for labs)
5 credits - some language classes that require a large amount of time in class</p>
<p>Most colleges require that you take at least 12 credits per semester to be considered a full time student. That could be either three or four courses. Some have an upper limit (Oberlin limits you to 16 per semester), while others let you take on as many as you’d like. So someone can be taking 20 credit hours, and really be taking only four courses, while someone with 18 could be in six. </p>
<p>I’d say the average student takes four courses each semester, though five is reasonable.</p>